1. Field of the Invention
The invention relates to circuit layout, and in particular relates to a circuit layout method and a layout circuit with combined tie cells.
2. Description of the Related Art
After design engineers (hereinafter referred to as ‘engineers’) place and route standard cells on a layout area, engineers usually prepares some spare cells on the layout area for adding more functions or changing design circuits after the chip tap out. However, these spare cells initially do not connect to any standard cells, and thus these spare cells should connect to tie-high cells or tie-low cells in order to avoid floating.
FIG. 1 shows a schematic diagram of one part of layout area 100 of an integrated circuit. There is no standard cell particularly shown on this part of layout area 100. Spare cells A and C are connected to tie-high cells 101 and 103 and spare cell B is connected to a tie-low cell 102. Thus, the voltages provided to spare cells A and C are tied at a high voltage by the tie-high cells 101 and 103, and the voltage provided to spare cell B is tied at a low voltage by the tie-low cell 102. In addition, the rest of the layout area 100 is filled by normal filler cells.
In some cases, using the engineering change order (ECO), engineers will change some functions of the chip after the chip tap out by replacing one of the standard cells with the space cell. FIG. 2 shows a schematic diagram of one part of layout area 200 of an integrated circuit before taking the engineering change order (ECO). Standard cells D and E, respectively corresponding to spare cells D′ and E′, are coupled to other standard cells (not shown in FIG. 2). Spare cells D′ and E′ are respectively coupled to tie-high cell D and tie-low cell E to avoid floating. In addition, the rest of the layout area 200 is generally filled by normal filler cells, such as capacitance filler cells.
After chip tap out, engineers may find that the operating of standard cells D and E falls short of their expectations, and therefore engineers use spare cells D′ and E′ to replace standard cells D and E. FIG. 3 shows a schematic diagram of one part of layout area 200 of an integrated circuit after taking the engineering change order (ECO). Spare cells D′ and E′, which now are “standard cells” D′ and E′, are directed to couple to other standard cells (not shown in FIG. 3) which are originally coupled to the standard cells D and E. The replaced standard cells D and E, which now become “spare cells” D and E, are respectively required to couple to the tie cells so as to avoid floating. However, the replaced standard cell D and E may be distant from their corresponding tie cells, thus causing routing congestion.